Not quite true, technically. SIS merely just stands for Shimano Index System. So on its face your statement that Accushift is the same spacing as SIS isn't accurate, since each speed in the Shimano indexing world has its own cog spacing standard. Sure Shimano had SIS on 6sp Dura-Ace but that kit is almost a unicorn. To the point that Sheldon Brown doesn't even reference Shimano SIS 6-speed spacing on his freewheel/cassette crib sheet. Shimano 7sp SIS spacing and every Shimano spacing standard from 8-11 are not compatible with Accushift or "normal" 6sp.

Be careful using the term Accu-7. Sheldon Brown refers to the two Suntour 7-sp standards as Ultra-7 (freewheel) and Microdrive-7 (cassette). They are different. Ultra-7 is the SAME as Shimano HG, IG, Sachs, SRAM, etc. at 5.0mm. Using the term Accu-7 just invites trouble because it isn't clear if you're talking about Ultra or Microdrive 7. However, if one were using a Centeron or floating upper pulley on their derailleur, I'd be shocked if the narrower cog spacing on Microdrive-7 even mattered. The pulley should correct the variance, I would think.